1. Field of the Invention
This invention relates in general to liquid crystal display devices, and more particularly, to display devices including cholesteric liquid crystals having a bistable capability and improved switching characteristics.
2. Discussion of the Background Art
In a background liquid crystal display device, a thin layer of liquid crystal material is interposed between light transparent substrates which are provided with delineated electrodes and alignment films on their inner confronting surfaces. Polarizing plates are also positioned adjacent to external surfaces. By selectively applying an electric field across the layer of liquid crystal material by address potentials applied to the electrodes, the transmittance of the liquid crystal device may be changed for displaying information in accordance with the addressed electrodes.
Several liquid crystal display devices using bistable cholesteric liquid crystals have been disclosed. For example, Japanese Patent H1-51818 discloses a liquid crystal display device with a plurality of liquid crystal cells which include a layer of cholesteric liquid crystals constituted to have an unstrained pitch approximately twice as large as a thickness of the liquid crystal layer.
Liquid crystal cells have a bistable character and can be switched between two states by the application of potential voltages to transparent electrodes, such that a twist angle of the liquid crystal layer along the layer thickness is approximately either 360.degree. or 0.degree., corresponding to a twisted state or a non-twisted (or uniform) state, each of which is hereinafter referred to as a twisted state or a uniform state, respectively. In addition, the liquid crystal cells are further provided with a pair of polarizing plates placed on respective top and bottom faces of the cell, thereby constituting a liquid display device.
When the polarizing plates are provided such that a transparency axis of one polarizing plate makes a right angle relative to a transparency axis of the other polarizing plate, and such that a direction of the liquid crystal alignment at the uniform state makes a 45.degree. angle relative to the transparent axis of each polarizing plate, birefringent colors appear due to the wavelength dependent nature of transmitted light through the polarizing plates.
These birefringent colors are generally not preferable for a display quality of black-and-white displays. Therefore, for liquid crystals at the uniform state with such a construction, which have an optical anisotropy .DELTA.n and a thickness of the liquid crystal layer d, an .DELTA.nd value may be adjusted to about 270 nm to satisfy retardation requirements for visible light, and to thereby acquire an approximately white display color.
In addition, although some birefringent effect may also arise in the twisted state, this does not significantly affect the nearly black display color quality due to relatively small values of the birefringence effect in the twisted state.
Display colors of nearly black and white quality can thus be achieved by background bistable liquid crystal display devices using cholesteric liquid crystals.
In order to implement driving of such liquid crystal devices with bistable cholesteric liquid crystals, at least two pulse voltages may preferably be applied to produce an electric field across a liquid crystal cell.
A first pulse is applied so as to have an amplitude large enough to bring the liquid crystals into a homeotropic state. A second pulse is subsequently applied with an amplitude less than that of the first pulse or with a zero amplitude, to cause a transition to either a twisted state or a uniform state depending on the amplitude and width of the pulse and to thereby accomplish modulation in optical transmittance or reflectivity, which can be utilized in display devices.
Because of an intrinsic property of the liquid crystal display, in that the modulation of optical transmittance of the device is implemented through the transition between the states described above, the display quality of the device, in general, tends to be affected considerably by temperature. For instance, although the device can be operated satisfactorily at room temperature under certain driving conditions, the device often suffers, at lower temperatures, from a decrease in display contrast or sometimes from operation failures even under the same driving conditions.
Although it is known that display quality of a liquid display device is relatively easily affected by temperature, this is particularly noticeable for a case of liquid crystal devices with bistable cholesteric liquid crystals. FIG. 4 illustrates a relationship experimentally obtained between temperature and thickness of a liquid crystal layer (or cell spacing), including the preferable range of these values for successful device operation.
As shown in FIG. 4, a desirable cell spacing which has been adjusted once to comply with driving conditions at room temperature may not be satisfactory at lower temperatures, since the desirable spacing is deviated from the aforementioned range preferable for device operation.